INTRODUCTION The objective is to test the validity of a tool allowing an offline measurement of the fraction of expired nitric oxide (FENO). The device is a T-tube on which a pressure gauge allows the control of the expiratory flow and whose two side branches have a gauge such as the bags assembled on each one of them fill successively. METHODS The first phase aims to check that the sample collected in the second bag answers the criteria of analysis of NO during a single expiration and that this measurement can be delayed. The second phase aims to test the feasibility and the repeatability of the offline analysis in children. RESULTS The device makes it possible to stabilize the expiratory flow at 100 ml/s. The NO concentration in the second bag is stable during 6 hours. The intra measurement coefficient of variation of delayed FENO 0.1 is 7% (N = 19). CONCLUSION A off line measurement of the exhaled nitric oxide is reliable in asthmatic children.